1. Technical Field
The present invention relates to a signal detection circuit that detects a signal according to an amplitude, a phase, and a frequency of an oscillation, and particularly, to a technique for detecting a signal according to an amplitude, a phase, and a frequency of an oscillation of a probe.
2. Description of the Related Art
Scanning probe microscopes or SPMs are microscopes that detect an interaction (for example, a tunnel current or a force of interaction) between a probe and a sample by bringing the probe closer to the sample to obtain information distribution, for example, on roughness on the surface of the sample and the electric potential distribution.
Atomic force microscopes or AFMs that are one kind of the SPMs are known. The AFMs detect a force of interaction between a probe and a sample to obtain a topographic image on the surface of the sample. More specifically, the AFMs perform feedback control on a distance between a probe and a sample to maintain a constant force of interaction between the probe and the sample. The AFMs include a cantilever with a sharp probe at its end as a force detector. By bringing the probe closer to the sample, the cantilever is displaced by the force of interaction between the probe and the sample. The force of interaction is detected using the amount of displacement. This type of AFMs is called contact mode AFMs or static mode AFMs.
On the other hand, AFMs in which a cantilever is mechanically excited at a frequency closer to the resonance frequency are called dynamic mode AFMs. The dynamic mode AFMs detect a force of interaction between a probe and a sample from change in amplitude, phase, or frequency of an oscillation that is caused by the force of interaction. The AFMs that detect a force of interaction using the amplitude, phase, and frequency are called an amplitude modulation AFM (AM-AFM), a phase modulation AFM (PM-AFM), and a frequency modulation AFM (FM-AFM), respectively.
The AFMs include, an amplitude/phase/frequency detection circuit (signal detection circuit) for detecting an amplitude, a phase, and a frequency. Conventional signal detection circuits include (i) a multiplying phase comparator including a low-pass filter (LPF) and (ii) a subtracting phase comparator including a delay circuit and a π/2 phase-shift circuit (see International Publication WO2010/023811).